Studies have continued on the characterization of monoclonal antibodies directed against complement component C9 and polyclonal Ab directed against neo-antigenic determinants of the membrane attack complex of complement C5b-9. These studies have demonstrated that only a small percentage of the Mab within ascites fluid was specific for C9, and this antibody was of low affinity. For these reasons, attempts are now underway to produce new clones recognizing C9. Studies with polyclonal antibody to neoantigens of the membrane attack complex have demonstrated low level but consistent cross-reactivity by ELISA of this antibody with an undefined constituent of serum. For these reasons polymerized C9 (poly C9) was prepared, and antibody directed against neoantigenic determinants within poly C9 was produced. This anti-poly C9 antibody has been thoroughly evaluated, and is known to react only with determinants on polymeric C9. Studies are underway with this antibody, looking for poly C9 deposition in the tissues and bloodstream of patients with various inflammatory and autoimmune disorders. The anti-poly C9 antibody is also being used to characterize poly C9 formation on bacteria and nucleated cells during the process of complement attack.